Synthesis of 2D layered transition metal (Ni, Co) hydroxides via edge-on condensation

Layered transition metal hydroxides (LTMHs) with transition metal centers sandwiched between layers of coordinating hydroxide anions have attracted considerable interest for their potential in developing clean energy sources and storage technologies. However, two-dimensional (2D) LTMHs remain largely understudied in terms of physical properties and applications in electronic devices. Here, for the first time we report > 20 μm α-Ni(OH)2 2D crystals, synthesized from hydrothermal reaction. And an edge-on condensation mechanism assisted with the crystal field geometry is proposed to understand the 2D intra-planar growth of the crystals, which is also testified through series of systematic comparative studies. We also report the successful synthesis of 2D Co(OH)2 crystals (> 40 μm) with more irregular shape due to the slightly distorted octahedral geometry of the crystal field. Moreover, the detailed structural characterization of synthesized α-Ni(OH)2 are performed. The optical band gap energy is extrapolated as 2.54 eV from optical absorption measurements and the electronic bandgap is measured as 2.52 eV from reflected electrons energy loss spectroscopy (REELS). We further demonstrate its potential as a wide bandgap (WBG) semiconductor for high voltage operation in 2D electronics with a high breakdown strength, 4.77 MV/cm with 4.9 nm thickness. The successful realization of the 2D LTMHs opens the door for future exploration of more fundamental physical properties and device applications.


Section 1. Testification of 2D growth mechanism through key parameter tuning of α-Ni(OH)2 synthesis
In order to maximize the 2D domains, the rate of crystal growth over nucleation first needs to be amplified.With slower nucleation, the Ni 2+ ions stay dissolved as [Ni(H2O)6] 2+ units without prematurely crashing out as polycrystalline or amorphous solid with bulk morphology.In an ideal case where nucleation is minimized, promoting growth in the 2D plane is the next target.
With the above understanding, our strategy of synthesizing large 2D LTMHs emphasizes promoting ab in-plane isotropy, which involves systematic control of parameters including cooling rate, soaking temperature, starting pH, as detailed in the following.
We first investigate the cooling rate on the morphology of the Ni(OH)2.We performed the synthesis at 120 °C under the cooling rate of 0.5, 1.5 and 3.0 °C/min.At 3.0 °C/min.We observed many random shaped flakes with rough surfaces (Fig. S12 A), on which islands and aggregates formed.As the cooling rate decreases to 1.5 °C/min (Fig. S12 B), circular thin flakes are easily found with average domain size of ̴ 20 μm, which is an exciting new record for 2D LTMHs synthesis.Interestingly, when the cooling is further slowed down to 0.5 °C/min (Fig. S12 C), the average domain size of obtained flakes is significantly smaller (around 1-2 μm) and thicker.The reason is that when cooling rate is decreased, the precipitation of seed crystals is largely slowed down and crystal growth is significantly encouraged.Thus, edge-on condensation happens rapidly on very limited amounts of seed crystals, overcoming the surface saturation of OH -and leading to a disordered interlayer add-on, eventually built 3D crystals and limited the size on intra-planar dimension.Moreover, PXRD results show no distinguishable difference among the Ni(OH)2 synthesized from different cooling rates (Fig. S12 D), suggesting cooling rate only subtly changed domain size and surface morphology without altering the crystal structure.
We further investigate the effect of the pH on the synthesis, since Ni(OH)2 is reported preferably formed and stable in the pH range of 9-13. 1 The sealed autoclave vessel used in the hydrothermal synthesis prevents the in situ monitoring of pH over the reaction, thus, we only control the initial pH of the solution and evaluate trends in outcomes.The initial pH value is adjusted from 5.57 to 7.60 by adding potassium hydroxide (KOH) while the cooling rates is fixed at 1.5 °C/min and the reaction temperature at 120 °C.The optical images show that the optimal 2D morphology is produced when no KOH is added, which reliably corresponds to an initial pH of 5.57 (± 0.2) (Fig. S13 A).With raised pH, thin circular flakes decrease in size and abundance (Fig. S13 B).Until pH=7.60,only small Ni(OH)2 particles arranged into amorphous films are observed (Fig. S13 C).Without extra KOH addition, the decomposition of urea provides gentle conditions in which OH -anions are generated steadily, promoting crystal growth and reducing the chance of nucleation.In this way, the solvated [Ni(H2O)6] 2+ have more time to spend in solution and have more opportunity to incorporate to existing nuclei and crystals (Fig. 2A II), rather than rapidly precipitating as particles.In contrast, the extra addition of OH -from KOH results in the supersaturation of Ni(OH)2 in the solution and promotes fast precipitation of amorphous solid Ni(OH)2 instead of intra-planar growth.Furthermore, the introduction of extra OH -species slows down the urea hydrolysis, resulting in less production of ammonia, and consequently less promotable to form α phase, as ammonia has widely been identified as an intercalation species in Ni(OH)2. 2 The PXRD clearly shows a transformation from mostly α-phase to multi-phase to β-phase Ni(OH)2 when increasing the pH (Fig. S13 D The increased integral intensity of (001) plane from β-phase implies the presence of in-plane growth, however, smooth 2D morphology is not observed upon transition to the β-phase.
Another important factor we investigated is the soaking temperature.We perform the synthesis at various temperatures from 80 °C to 100 °C, 120 °C, 140 °C and 160 °C, where the cooling rate is set as a constant, 1.5 °C/min.At 80 °C (Fig. S14 A), only amorphous and bulky aggregates are observed, which persists when the temperature is elevated to 100 °C (Fig. S14 B).We believe that under low temperatures, the autogenous pressure generated in the fixed volume is too low to provide high supersaturation for [Ni(H2O)6] 2+ units to form and add onto the seed crystals.Accordingly, as the temperature increases to 120 °C (Fig. S14 C), large circular flakes are observed.However, at even higher temperatures (e.g., 140 °C, Fig. S14 D), surface defects, holes, and crimps are more widely observed in the optical image.This is due to the increased production of free ammonia from promoted urea hydrolysis.The excess ammonia forms [Ni(NH3)6] 2+ complexes, effectively consuming Ni 2+ ions and preventing the formation of [Ni(H2O)6] 2+ units.Different from H2O, the deprotonation hardly happens for NH3, thus, the seed crystals lose attraction to [Ni(H2O)6] 2+ units, stopping the growth of the crystal.Notably, with the temperature further increasing to 160 °C (Fig. S14 E), the average size decreases to ̴ 5 μm and some bulky aggregates are found gathering around the circular flakes, which is another consequence of nuclei edge blocking.The higher the temperature is, the more [Ni(NH3)6] 2+ and less [Ni(H2O)6] 2+ complexes are formed, then the more terminal groups on the existing nuclei and crystals are occupied by NH3.Thus, edge substitution cannot proceed to achieve intra-planar growth, leading to smaller crystal size eventually.
PXRD spectra of samples synthesized under different temperatures is shown in Fig. S14 F. The peak from α-(100) at 18.3 ° (indicated by the green arrow) decreases significantly when increasing the temperature from 120 to 160 °C, suggesting intralayer growth along (100) direction is diminished.This result matches with the decreasing average domain size observed .Moreover, as the temperature increased, the β (001) peak at 21.7 ° (grey arrow) becomes smaller and less visible, indicating the decreased proportion of β phase space in the α/βinterstratification structure, which is the consequence of interlayer swelling under high temperature, suggesting high temperature would produce purer and highly ordered α-Ni(OH)2.
Besides, we also observed a set of small peaks (purple arrow) along the 140 °C spectrum (Fig. S14 G) in the range of 30 to 80°, suggesting the disorder and rearrangement of the crystal structure due to an ongoing phase transition at this intermediate temperature.
In a nutshell, the comprehensive investigation of key parameters on the synthesis supports our hypothesis on the edge-on condensation 2D growth mechanism.Briefly, the synthesis starts with the formation of seed crystals and octahedral [Ni(H2O)6] 2+ units, followed by continuous water and hydroxyl ligand-substitution.We also realize that controlling the moderate nucleation and growth rate, together with an acidic environment are the key to promote the 2D intra-planar crystal growth of Ni(OH)2 .

Fig. S4 .
Fig. S4.Chemical states of synthesized 2D Co(OH)2, detected by XPS.(A) Survey spectrum, (B) Co 2p spectrum, (C) O 1s spectrum of Co(OH)2 thin flakes, indicating the occurrence of Co and O elements at their binding energies, with a characteristic spin energy separation of 15.7 eV.The slight amount of CoO might be generated during synthesis or oxidized from Co(OH)2 during storage.

Fig. S7 .
Fig. S7.Ni(OH)2 morphology dependence on initial NH4OH-tuned pH of solution from (A) no additional NH4OH, (B) 0.2, (C) 0.6, and (D) 0.8 ml additional NH4OH.The domain size decreases with the increasing NH4OH, which will cease the 2D crystal expansion based on our proposed mechanism.

Fig. S8 .
Fig. S8.Chemical states of synthesized 2D Ni(OH)2, detected by XPS.(A) Survey spectrum, (B) O 1s spectrum of Ni(OH)2 thin flakes, indicating the occurrence of Ni and O elements at their binding energies and the appearance of H2O, mainly as intercalation between layers to form α-Ni(OH)2.Besides Ni and O signals from Ni(OH)2 labeled in solid black, other extrinsic elements like C, Si and Cu are also found and labeled in dark grey in (A).The presence of C is normal in XPS examination since it is always part of the environment and it hardly can be excluded even under ultra-high vacuum.Moreover, Si is from the sample substrate and Cu is

Fig. S9 .
Fig. S9.SAED pattern of 2D Ni(OH)2, detected by TEM.(A) Simulated diffraction of lattice planes within (220) from Stem-Cell.(B) Collected diffraction of lattice planes within (220).(C) Collected diffraction at a different position on the same flake.(D) Collected diffraction at a different position on the same flake, also is shown in Fig. 3C as inset.

Fig. S13 .
Fig. S13.Ni(OH)2 morphology dependence on initial KOH-tuned pH of solution from (A) 5.57 (optimal condition, no KOH added) to (B) 7.60, (C) 7.22, while the soaking temperature is 120 o C and the cooling rate is 1.5 °C/min.(D) PXRD spectrum of Ni(OH)2 synthesized with different starting pH, suggesting more β phase is formed with extra KOH addition.Peak labeled by "*" is generated from the stacking faults.